Extracts containing tissue-specific DNA-inhibitory activity were prepared from normal FL (Swiss) and BALBc3T3 cells, from these cells transformed with sarcoma virus and from revertants cloned from the transformed cell lines. By testing all extracts on all cell lines we found that (1) production of and susceptibility to the inhibitors were decreased in transformed BALB/c cells (2) specificity varied with expression of the transforming genome, as an extract from a given cell line inhibited the growth of its cell of origin, e.g. revertant, more than normal or transformed cells, and (3) there was also a DNA-synthesis stimulator
